1. Field of the Invention
The present invention relates to hydrogenated polybutadiene comprising monomeric units of 1,4-butadiene and 1,2-butadiene addition products. More particularly, it relates to polybutadiene comprising hydrogenated blocks or segments of monomeric units of 1,2-butadiene and 1,4-butadiene addition products containing at least one crystallizable segment or block comprising an average of at least about 10 weight percent of the total hydrogenated copolymer chain, and at least one low crystallinity segment or block. The instant invention also relates to oleaginous compositions containing said copolymers as viscosity index improver additives, a process for making these copolymers and a method to control the viscosity of oleaginous compositions.
2. Description of Related Art
Various copolymers of butadiene with other olefins are known to be used as oil additives. These include hydrogenated copolymers of butadiene with another conjugated diene such as isoprene. The copolymers are disclosed to be random or block copolymers. The following review of the related art illustrates attempts which have been made to develop hydrogenated butadiene base copolymers for use as an oil additive including a viscosity modifier.
U.S. Pat. No. 4,804,794 discloses segmented copolymers of ethylene and at least one other .alpha.-olefin monomer. Each copolymer is intramolecularly heterogeneous and intramolecularly homogeneous. At least one segment of the copolymer constituting at least 10% of the copolymer chain, is a crystallizable segment. The remaining segments of the copolymer chain are termed low crystallinity segments, and are characterized by an ethylene content of not greater than about 53 weight percent.
The .alpha.-olefin can include those containing 3 to 18 carbon atoms. .alpha.-olefins having 3 to 6 carbon atoms are indicated to be preferred. With the most preferred copolymers being copolymers of ethylene with propylene or ethylene with propylene and diene. The copolymers are disclosed to improve the properties in oleaginous fluids, in particular lubricating oil.
U.S. Pat. No. 3,419,365 discloses hydrogenated copolymers of butadiene and styrene as pour point depressants for distillate fuel oil; U.S. Pat. No. 3,393,057 discloses polymers of butadiene C10 to C24 normal alpha-monoolefins and styrene or indene as pour point depressants for fuel and lubricating oils; and U.S. Pat. No. 3,635,685 discloses pour point depressants comprising hydrogenated butadiene-styrene copolymers which contain a hydroxy, carboxy, or pyridyl terminal group.
U.S. Pat. No. 3,312,621 discloses polymers of conjugated diolefins which are predominantly in the 1,4-addition configuration, as viscosity index (V.I.) improvers. Butadiene, isoprene, 1,3-pentadiene, and copolymers of such diolefins are specifically disclosed as suitable.
U.S. Pat. No. 3,600,311 discloses viscosity index improvers of hydrogenated homopolymers of butadiene in which about 45 to 95% of the butadiene monomers are in the 1,4-configuration.
U.S. Pat. No. 3,795,615 discloses viscosity index improvers of hydrogenated copolymers of butadiene with a different conjugated diene, e.g., isoprene in which the monomer units in the polymer are predominantly in the 1,4-configuration. This patent discloses that hydrogenated 1,4-polybutadiene is not useful as a viscosity index improver, since the final product is an oil-insoluble polyethylene. Hydrogenated polybutadienes with an appropriate amount of 1,2-structure would give the necessary solubility and would have viscosity index improving properties. However, this references teaches that it is necessary to very precisely control the amount of 1,2-structure formed. If an inadequate amount of 1,2-structure is formed, the polymer is not sufficiently soluble; if too much 1,2-structure is formed, the polymer is not as effective in increasing the viscosity index. The patent further discloses that when polybutadiene is anionically polymerized to the high degree of polymerization needed for viscosity index improvers, it is very difficult to obtain precise control of the amount of 1,2-addition product by variations in reaction conditions.
U.S. Pat. No. 3,965,019 discloses hydrogenated random, tapered or block copolymers of butadiene and isoprene to be useful as viscosity modifiers.
U.S. Pat. No. 4,032,459 discloses a viscosity index improver comprising a copolymer of butadiene and isoprene having between 20-55% 1,4-configuration, this polymer then having been hydrogenated to remove substantially all of the olefinic unsaturation.
U.S. Pat. No. 4,073,737 discloses viscosity index improver comprised of a hydrogenated copolymer produced by copolymerization of from about 1 to about 10 mole percent butadiene, at least one other C5 to C12 conjugated diene, and up to 45 mole percent of a vinyl aromatic monomer.
A useful class of viscosity index improvers for lube oil compositions are star-shaped polymers comprising a nucleus such as divinylbenzene with polymeric arms linked to it. Such polymers are disclosed in patents, such as U.S. Pat. Nos. 4,358,565 and 4,620,048. Generally star-shaped polymers are disclosed to be formed by polymerizing one or more conjugated dienes and optionally, one or more monoalkenylarene compound in solution in the presence of an ionic initiator to form a living polymer. For the purpose of the present invention the term "living polymer" is used consistent with Billmeyer, Textbook of Polymer Science, 2d Ed., page 348, Wiley-Interscience, John Wiley and Sons, page 318 (1971). Specific conjugated dienes include conjugated dienes having 4 to 12 carbon atoms and optionally, one or more monoalkenylarene compounds. Typical and useful conjugated dienes include butadiene (1,3-butadiene) and isoprene. The living polymers thereby produced are then reacted with a polyalkenyl coupling agent to form star-shaped polymers The coupling agents have at least two non-conjugated alkenyl groups. The groups are usually attached to the same or different electron drawing groups, e.g., an aromatic nucleus. Such compounds have the property that at least two of the alkenyl groups are capable of independent reaction with different living polymers, and in this respect, are different from conventional dienes polymerizable monomers, such as butadiene and isoprene. The coupling agents may be aliphatic, aromatic or heterocyclic. Examples of aliphatic compounds include polyvinyl and polyallyl acetylenes, diacetylenes, phosphates, and phosphites, as well as the dimethacrylates, e.g., ethyl dimethacrylate. Examples of suitable heterocyclic compounds include divinyl pyridine and divinyl thiophene. Coupling agents disclosed to be preferred in the U.S. Pat. No. 4,358,565 patent are polyalkenyl aromatic compounds with the most preferred being indicated to be polyvinyl aromatic compounds. Examples of such compounds include those aromatic compounds, e.g., benzene, toluene, xylene, anthracene, naphthalene, and durene which are substituted by at least two alkenyl groups, preferably directly attached thereto. Specific examples include polyvinyl benzenes, e.g., divinyl, trivinyl, and tetravinyl benzenes; divinyl, trivinyl and tetravinyl, autho, meta and paraxylenes, divinyl naphthalene, divinyl ethyl benzene, divinyl biphenyl, diisobutenyl benzene, diisopropanol benzene, and diisopropanol biphenyl. The polyalkenyl coupling agent should be added to the living polymer after the polymerization of the monomers is substantially complete, i.e., the agent should only be added after substantially all of the monomer has been converted to living polymer.
The amount of polyalkenyl coupling agent added may vary, but preferably, at least 0.5 moles is used per mole of unsaturated living polymer. Amounts from 1 to 15 moles, and preferably 1.5 to 5 moles are preferred.
There exists a need for viscosity index improvers which, when added to oleaginous compositions, such as lube oil compositions, yield compositions exhibiting better or improved low temperature viscometric characteristics than are obtainable by the use of conventional viscosity index improver additives. The copolymers of the instant invention provide oleaginous compositions exhibiting such improved low temperature viscometric characteristics, and additionally, improved shear stability.
For convenience, certain terms that are repeated throughout the present specification and claims are defined below.
a. Viscosity Index (V.I.) is the ability of a lubricating oil to accommodate increase in temperature with a minimum decrease in viscosity. The greater this ability, the higher the V.I.
b. A block copolymer is a copolymer having at least one sequence (also referred to as block or segment) of the same monomer units. Each sequence has at least two monomer units. Block copolymers typically have a plurality of each type of monomer making up the copolymer. The terms relating to block copolymer are consistent with those given in Billmeyer, Jr., Textbook of Polymer Science, Second Edition, Wiley-Interscience (1971). For the purpose of the present invention the term block or segmented copolymer includes a copolymer comprising monomeric units of 1,2-butadiene and 1,4-butadiene.
c. Average methylene content of a segment of the hydrogenated copolymer is the average number of methylene moieties or units present in the segment. The methylene units are those present in the particular segment as a result of the polymerization and hydrogenation of butadiene and at least one other conjugated diene, such as isoprene. Thus, for example, the hydrogenation product of the 1,4-addition product of two 1,4-butadiene molecules contains one methylene segment comprised of eight methylene units and has a methylene content of 100%, i.e., --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 CH.sub.2 --CH.sub.2 --. The hydrogenation product of the 1,2-addition product of 1,2-butadiene contains one methylene units alternating with an ethylene substituted methylene unit, i.e., ##STR1##
The following defines how percent methylene units are calculated from the percentages of hydrogenated 1,2 and 1,4 polybutadiene in the polymer. Each enchained 1,4-butadiene (B.sub.1,4) unit contributes four methylenes. Each 1,2-butadiene contributes one methylene and one substituted methylene. Where B represents the mole fraction 1,4-butadiene then (1-B) is the fraction 1,2-butadiene. The mole fraction methylenes present, X.sub.CH.sbsb.2 is then ##EQU1## where B.sub.1,4 is the fraction of 1,4-addition of butadiene. For B.sub.1,4 =1, X.sub.C.sbsb.2 =1. For all 1,2-addition X.sub.CH.sbsb.2 =0.5, but there can be no long methylene sequences.
To obtain crystallizable sequences there must be sequences of 1,4 enchained units. For random addition of 1,2-butadiene and 1,4-butadiene units B.sub.1,4 is approximately 0.3 or 30 percent to obtain crystallinity. From formula (1) X.sub.CH.sbsb.2 =0.73 at B.sub.1,4 =0.3. In principle, if the statistical distribution of 1,2 and 1,4-butadiene additions is of a block type, than random crystallinity could occur at B.sub.1,4 below 0.3.
In all cases where sequences of methylenes are long enough to crystallize at least two adjacent hydrogenated 1,4-butadiene units are present, and generally 3 or more are present. In such cases, methylenes from 1,2-butadiene becomes less important for crystallization since they add only one or two units to an 8 to 12 unit sequence. For purposes of the present invention the composition of crystallizable segments is in terms of 1,4-butadiene content, i.e, only those methylene segments contributed by 1,4-butadiene units are counted.
d. By "crystallize" it is meant that the methylene sequences in the polymer associate into ordered state consistent with the classical definition of polymer crystallinity as set forth, for example, by Flory, Principles of Polymer Chemistry, Cornell University Press (1953).
e. Crystallizable units are defined as methylene groups in sequence which exhibit a heat of fusion when measured by differential scanning calorimetry (DSC) upon cooling from the melt. In a procedure useful for the present invention a sample can be formed into an approximately 0.030 inch thick sheet for 30 minutes at 150.degree. C. and then annealed at 20.degree. C. for 48 hours prior to measurement, loaded into the calorimeter at that temperature, rapidly cooled to -100.degree. C. and scanned to 150.degree. C. at 20.degree. C./minute. Only sequences melting between 20.degree. C. and 140.degree. C. are included.
f. The weight percent crystallizable units is: ##EQU2##
On this basis pure melt crystallized polymethylene of high Mw (i.e., where the end groups effects are not significant; typically greater equal or about 20,000) has about 60% crystallizable units. Kinetic restrictions prevent them from all crystallizing. Percent crystallinity can be measured by a techniques, as defined in G. Ver Strate, Z. W. Wilchinsky, J. Pol. Sci. Physics, A2, 9, 127 (1971), which is incorporated herein by reference. The degree of crystallinity measured is a function of the sample's annealing history. Some low amount is desirable in this product when the sample is annealed at 20.degree. C. for more than 48 hours after preparation of a void-free, strain-free specimen by heating to 150.degree. C. for 30 minutes in a suitable mold. Crystallizability also depends on other factors: temperature, diluent, and the composition of the copolymer.
g. The association temperature (T.sub.a), is the temperature at which crystallization of the copolymer of the present invention can be determined by studying the temperature dependance of the relative viscosity (.eta.rel). Deviation from an established (d.eta.rel/dT) trend occurs when significant association at polymer segments due to crystallinity starts. (ASTM method D-445 for kinematic viscosity can be run at a series of temperatures. The polymer concentration in these measurements should be the same as that in the formulated oil, for example, 1%).
h. The cloud point temperature is the temperature (T.sub.c) at which crystalline clouds (turbidity) are first observable upon cooling of oil when tested according to ASTM D-2500. The cloud paint temperature can be correlated with the association temperature.
i. A crystallizable segment of the hydrogenated copolymer chain is rich in methylene units, with an average methylene content of at least about 75 mole percent. The methylene content depends on the monomers used to prepare the polymer and the nature of their inclusion in the polymer. The methylene units will crystallize at a given temperature and concentration in solution only if they are in long enough sequences, with only a limited number of interruptions due to substituted methylene units. More interruptions are acceptable with methyl substitutions than with larger groups since methyl groups can enter the polymethylene crystal lattice. Methylene units can be identified by C.sub.13 NMR, T. Hayashi, Y. Iroue, R. Chujo, Macromolecules, 21, 3139, 1988, and references therein. Sequences of 5 or more cannot be distinguished. However units are actually crystallizable only if they are present in sequences of about 13 methylenes or longer.
j. A low crystallinity segment has an average methylene content less than about 75 mole percent, and is characterized in the unoriented bulk state after at least 24 hours annealing by a degree of crystallinity of less than about 0.2% at 23.degree. C.
k. Molecular weights of the hydrogenated copolymer were measured by a combination of gel permeation chromatography and on line laser light scattering and described in G. Ver Strate, C. Cozewith, S. Ju, Macromolecules, 21, 3360, 1989. The specific refractive index increment in trichlorobenzene at 135.degree. C. was assumed to be -0.104 cc/g for all hydrocarbon structures.